Thermal fuse utilizing an electrically conductive pellet



Sepf- 20 1966 G. F. EBENSTEINER THERMAL FUSE UTILIZING AN ELECTRICALLYCONDUCTIVE PELLET Filed April 7, 1965 i v VW a Il Q ,QN v Q \\P Ev MYNVM Nm. Qw mm., NN

United States Patent Ofitice 3,274,361 Patented Sept. 20, 1966 3,274,361THERMAL FUSE UTILIZING AN ELECTRICALLY CONDUCTIVE PELLET Gerald F.Ebenstciner and Gregory J. Rusinyak, Minneapolis, Minn., assignors, bymesne assignments, to United-Carr Incorporated, Boston, Mass., acorporation of Delaware Filed Apr. 7, 1965, Ser. No. 446,225 4 Claims.(Cl. 20D-123) This invention relates generally to circuit-breakingdevices, and pertains mo-re particularly to a thermal fuse employing atemperature-sensitive pellet of electrically conductive material.

There are a number of situations in which it is necessary or at lea-stdesirable that a circuit be opened when an excessive or abnormaltemperature is experienced. This is for the purpose of protectingtemperature-vulnerable equipment from damage in the region where theprotective device is installed, For instance, it may be necessary toafford adequate protection to an electrical device, such as a motor,whereby such device is decnergized when it becomes overheated. Also, asa further illustration, liquid fuel may be in the process of beingpumped and if a fire occurs, .it is essential that the pumping operationbe immediately discontinued.

Accordingly, one object of the present invention is to provide a thermalfuse that will be highly reliable in breaking the electrical circuit inwhich it is located. More specifically, the invention has for an aim theproviding of baflles that are instrumental in preventing the remaking orre-establishment of the electrical path once the excessive temperaturehas been reached. In this regard, a baille arrangement is incorporatedinto the fuse constructed in accordance witht-he teachings of thepresent invention so that there cannot be a bridging back by the moltenfuse material.

Another object of the invention is to provide a theral fuse of theforegoing character that can be oriented in any position. Consequently,it is not necessary to exercise care in installing the thermal fuse, forit will function irrespective of the particular attitude in which it ismounted.

A further object is to provide a thermal fuse that can be connecteddirectly in the primary circuit of the device that it is to control, itnot being necessary to employ any relays. Stated somewhat differently,the invention envisages a thermal fuse that can carry a relatively highelectrical current.

Still another object of the invention is to provide a thermal fuse thatcan be assembled easily and which does not require close tolerancesother than those needed for affording the requisite amount of bafflingaction for the degree of reliability that is demanded by the particularinstallation. More specifically, the longitudinal dimensions are notcritical, although the radial dimensions must be controlled within theranges demanded for the particular degree of balllng that is to beprovided.

Yet another object of the invention is to provide a thermal fuse thatwill be inexpensive to manufacture by virtue of the simple constructionof the component parts constituting the structure.

These and other objects and advantages of our invention will more fullyappear from the following description, made in connection with theaccompanying drawing, wherein like reference characters refer to thesame or similar parts throughout the several views and in which:

FIGURE l is a longitudinal sectional view taken through the center of athermal fuse exemplifying the invention, the view showing the fuse priorto the fusible pellet becoming molten, and

FIGURE 2 is a longitudinal setcional view corresponding to FIGURE l butillustrating the position of the parts after the pellet has reached apredetermined ternperature which causes it to melt and allow the circuitto be interrupted.

Referring in detail to the drawing, the thermal fuse there depictedcomprises a metallic casing or shell having a closed end 12 and -acylindrical side wall 14. The cylindrical side wall 14 is formed with ashoulder at 16 and has an inturned or crimped flange 18 at the endthereof remote from the closed end -12.

Contained within the casing 10 adjacent the closed end 12 is a fusiblepellet of an appropriate electrically conductive material for themaximum ambient temperature that is permissible for the device that isto be protected. For instance, if the permissible temperature is on theorder of C., then indium proves very satisfactory. Of course, variousalloys can be utilized and thereby obtain a circuit interruption at theproper temperature.

A movable contact element has been denoted generally by the referencenumeral 22 and is formed with a flat head 24 and an integral shank 26. Asleeve member designated generally by the numeral 28 has a tubularbaille portion 30` encircling the shan-k 2-6, there being preferablyonly a sufficient amount of clearance between the shank and portion 30to permit free sliding movement of the shank relative to the portion 30when the pellet 20 beco-mes molten. The sleeve member 28 also includes aradially extending flange 32 that forms a shoulder between the tubularbaflle portion 30 and a larger diameter ltubular portion 34. Stillfurther, at the free end of the larger diameter portion 34 is anoutwardly directed flange 36 that abuts against the shoulder 16 of thecylindrical side wall 414. Actually, the cylindrical portion 34 ispress/fitted into the cylindrical side wall 14 and itis the flange 36that longitudinally positions the sleeve member 28 within the casing 10.An annular knife edge 38 is formed on the flange 36 and performs afunction described below.

-The larger diameter tubular portion 34, which is somewhat cup-shaped,accommodates therein a rubber bushing 40 formed with a smaller boreportion 42 and a larger bore portion 44. At this time, it will beperceived that during assembly the annular knife edge 3-8 cuts throughpart of the rubber bushing 40 s-o as to provide what amounts to adetached annular flange 46 that provides an effective moisture seal atthis end of the casing 10.

Describing now the specific configuration of the xed Contact element,which has been identified by the reference numeral 48, it will be seenthat this element constitutes a plurality of spaced spring 4fingers 50that are integral with a shank 52.

Circumscribing the shank 52 and projecting somewhat from the right endof the casing 10 is a high temperature insulator 54 which can be ofappropriate plastic or cerramic material. Thus, during assembly, theflange 18 is crimped or spun against the insulator 54 so as to preventdetachment of the parts contained within the casing 10.

An additional sleeve member 56 -has a tubular barrier portion 57 and aradially directed flange 58. The tubular barrier portion 57 encirclesthe previously-mentioned tubular barrier portion 30 and has only aslight clearance with respect thereto which allows the sleeve member 56to move relative to the sleeve member 28. ln other words, there is onlya sliding fit between the tubular barrier portions 30 and 57.

A coil spring 60 has one end thereof bearing against the flange 58 andits opposite end bearing against the flange or shoulder 32.Consequently, the coil spring 60 biases the movable contact 22 againstthe fusible pellet 20 and in a direction such as to cause the shank 26to be urged in a direction away from the fixed contact element 48. Morespecifically, the free end of the shank 26 is only frictionally engagedwith the contact element 48 via the spring lin-gers 5t). When tlhepellet 20 becomes suiciently molten, then the shank 26 moves to the leftunder the influence of the coil spring 40 so as to separa-te the contactelements 22 and 48.

It will be noted that the enlarged head 24 on the movable contactelement 22 has a diameter somewhat lless than the dia-meter of the boreof the casing 10. Therefore, an annular space exists which functions asa pasw sage for the ow of the material when it becomes uid due to asuiciently high temperature. The space to the right of the head 24 formsa chamber or compartment 62 of suficient size to accommodate all of thematerial constituting the pellet 20. rThus, the material forming thepellet 2()` is displaced from its original position and flows into thechamber 62. 'From what has been described, it will be manifest that atortuous path is pre sented to Ithe owing material by reason of theclose t between :the barrier port-ions and 57` Even if any moltenmaterial does enter between these barrier portions, it still musttraverse a very small clearance path between `the shank 26 and thebarrier portion 30. Sti-ll further, the fact that the rubber bushing 40'has a -smaller bore portion 42, which when not distended can be slightlysmaller than the cross section of the shank 26, presents still a furtherresistance to any material entering into the larger bore portion 44 ofthe bushing 40. Therefore, it is virtually impossible for any of thematerial of which the pellet 20 is composed to pass into the regionbetween the contact elements 22 and 48. In this way, assurance is giventhat there will not be a bridging back which will re-establish theelectrical path that `is normally formed between the contact elements 22and 48.

Although not important lto a practicing of the invention, the drawingshows a rst conductor 64 having a buttonlike head or flange 66 thereonwhich is welded or ybrazed to the closed end 12 of the casing 10.Somewhat similarly, a second conductor 68 is attached or connected tothe shank 52 of the lixed contact 48, a portion of .the conductor 68being encompassed by the insulator 54.

In use, the conductors 64, 68 are connected in circuit with theelectrical device to =be protected or shut down, as the case may be, andwhen the pellet 20 melts due to it reaching a prescribed temperature,then the biasing action of the spring 60 takes over so as to urge themovable contact element 22 to the left. In other words, the movablecontact element 22 shifts from the position in which it is pictured inFIGURE 1 to that illustrated in FIG-URE 2, thereby separating ordisengaging the contact ele-ment 22 from the contact element 48. As canbe discerned from FIGURE 2, the shank 26 is no longer within thecontines of the spring lingers and the circuit is completely broken.

Although not mentioned herein, it will be appreciated that the thermalfuse constituting the present invention can have the various partsthereof appropriately plated.

Therefore, Whereas the thermal fuse normally passes current from oneconductor 64 to the other conductor 68, doing so through the pellet 20,it does not do this when Ithe pelle-t 20 thas melted. Consequently,complete assurance is provided that the thermal fuse structure willperform its safeguarding role.

It Iwill, of course, be understood that various changes may be made inthe form, details, arrangements and proportions of the parts withoutdeparting from `the scope of our invention as set forth in the appendedclaims.

What is claimed:

1. A thermal fuse comprising an elongated casing, a fusible -pellet ofelectrically conductive material disposed within said casing at one endthereof, a fixed electrical contact element disposed within said casingat the other end thereof, a movable contact element having an enlargedhead engaging said pellet and a shank having its free end engaging saidfixed contact, .a fixed sleeve member having a tubular baille portionclosely encircling said shank, said shank being free `to slide relativeto said bafe portion, a movable sleeve member having a tubular batiieportion closely encircling said first bathe portion provided with aflange abutting said enlarged head, said second battle portion beingfree to slide relative to said lirst baffle portion, and a coil springacting against said flange so as tobias said movable contact elementtoward said pellet and away from said fixed contact to effect separationof said contact elements when said pellet becomes molten due to asuflicient rise in temperature.

2. A thermal fuse in accordance with claim 1 in which said lixed sleevemember is formed with an outwardly directed liange and an enlargedcup-shaped portion engaged Iwithin the interior of said casing adjacentthe end thereof containing said fixed contact element, the other end ofsaid coil spring abutting against sand last-mem tioned flange` 3. Athermal -fuse in accordance with claim 2 including a resilient bushingcontained in the cup-shaped por tion and having a lirst bore yportiontightly but releasably encircling said shank and a second bore portionencircling a portion of said lixed contact.

4. A thermal fuse in accordance with claim 3 in which Said fixed contactelement includes a plurality of spring fingers frictionally engagingsaid shank.

References Cited by the Examiner UNITED STATES PATENTS 513,796 1/1894Lewers 200-142 1,645,201 10/1927 McBrien 200-142 1,753,846 4/1930 Brand20D-142 2,883,492 4/1959 Landers 200-142 2,931,874 4/1960 LeamanZOO-61.08 X 2,934,628 4/1960 Masser et al. 20G-123- X 2,955,179 10/1960Milton et al 200-142 2,999,912 9/1961 Kincaid et el. 200-142 3,180,9584/1965 Mer-rill 20G- 142 BERNARD A. GILHEANY, Primury Examiner. H. B.GILSON, Assistant Examiner.

1. A THERMAL FUSE COMPRISING AN ELONGATED CASING, A FUSIBLE PELLET OFELECTRICALLY CONDUCTIVE MATERIAL DISPOSED WITHIN SAID CASING AT ONE ENDTHEREOF, A FIXED ELECTRICAL CONTACT ELEMENT DISPOSED WITHIN SAID CASINGAT THE OTHER END THEREOF, A MOVABLE CONTACT ELEMMENT HAVING AN ENLARGEDHEAD ENGAGING SAID PELLET AND A SHANK HAVING ITS FREE END ENGAGING SAIDFIXED CONTACT, A FIXED SLEEVE MEMBER HAVING A TUBULAR BAFFLE PORTIONCLOSELY ENCIRCLING SAID SHANK, SAID SHANK BEING FREE TO SLIDE RELATIVETO SAID BAFFLE PORTION, A MOVABLE SLEEVE MEMBER HAVING A TUBULAR BAFFLE